1. Field of the Invention
This invention relates to operation of ambient air-cooled heat exchangers, and more particularly to monitoring the operation of such heat exchangers during very cold weather.
Heat exchangers using ambient air as the cooling medium are used in many industrial processes. For example, the overhead stream from petroleum refinery fluid catalytic cracker main fractionators is often condensed and cooled by air-cooled heat exchangers. The overhead stream from these fractionators typically contains a substantial amount of water, such as from 5 to 10 weight percent. These heat exchangers may be single pass units having a large number of parallel tubes. In a typical unit, upflow induced-draft fans pull cooling air over the tube bundles, and some means of controlling the airflow, such as adjustable louvers, is generally used in conjunction with fan control to regulate the cooling.
In very cold climates, where ambient air temperatures reach -20.degree. C. and lower, there is a potential problem of freeze-up in the tubes due to the very cold air causing freezing of water in the tubes. This can lead to rupturing of a tube with serious consequences. Hydrocarbons leaking from a ruptured tube can present a serious fire hazard.
2. The Prior Art
Several methods have been considered or actually tried in an effort to eliminate the problem of tube freeze-up.
One proposed solution involved attaching thermocouples to the outside of the tubes which are subjected to the coldest air. The thermocouples are located near the outlet ends of the tubes as this was the coldest part of each tube. These thermocouples were connected to a monitor to provide operators with an indication of potentially freezing conditions. However, even when the thermocouples were heavily insulated, the readings obtained were unreliable and the technique was considered unsatisfactory.
Another proposed solution involved controlling the inlet air louvers in response to the bulk outlet temperature. This was not satisfactory as the relation between bulk outlet temperature and individual tube temperature was indefinite.
Another proposed solution involved partial air recirculation to hold air temperature above the freezing point. However, this would require extensive ductwork and control to provide the proper amount of warm outlet air for mixing with cold inlet air.
Still another proposal involved use of steam coils in the air inlet. This was rejected as expensive and energy inefficient.
Finally, it was proposed to install additional piping which permits shutting off a portion of the tubes during very cold weather. However, this would have required considerable work each time the temperature changed, and for that reason was rejected.
Thus, there has been a long-standing need for an improved method of controlling the operation of an air-cooled heat exchanger in very cold conditions.